This invention relates to lithography and more specifically to scanning beam lithography using beams that have a shaped cross section.
The field of lithography is well known, especially for use in the semiconductor industry. Typical is semiconductor fabrication by electron beam lithography or laser beam lithography where the electron beam or laser beam is scanned across a sensitive layer. This process is used to fabricate masks or to direct write semiconductor wafers. Lithography systems generate or expose patterns on a substrate which is typically the semiconductor wafer or mask blank by controlling the flow of energy (the beam) from a source to the substrate coated with a resist layer sensitive to that form of energy. Pattern exposure is controlled and broken into discrete units commonly referred to as flashes, wherein a flash is that portion of the pattern exposed during one cycle of an exposure sequence. Flashes are produced by allowing energy from the source, for example, light, electron, or other particle beams to reach the coated substrate within selected pattern areas. The details of flash composition, dose, and exposure sequence used to produce a pattern, and hence the control of the lithographic system, make up what is known as a writing strategy.
A traditional raster scan writing strategy employs a uniform periodic raster scan, somewhat similar to television raster scanning. A mechanical stage moves a substrate, for example, placed on a table, uniformly in a direction orthogonal to the direction of the uniform scan of the energy beam. In this manner, a pattern is composed on a regular grid with a regular scan trajectory resulting from the orthogonal movement of the stage and beam. When the beam is positioned over a grid site requiring exposure, the beam is unblanked and the underlying site exposed. In some embodiments, the amount of dose, or energy, at each site is varied as required. Hence, exposure data can be organized in a time order related to the regular scanned trajectory, and only the dose for each site need be specified. The distinguishing characteristics of a traditional raster scan writing strategy are a small round (Gaussian) beam, exposing sites one at a time, a periodic scan moving sequentially to each site of the grid, and a rasterized representation of data corresponding to the required dose for each site or xe2x80x9cpixelxe2x80x9d of the grid. By Gaussian is meant a beam that is most intense at the center and whose intensity falls off (to good approximation) in accordance with a Gaussian curve towards its perimeter.
Also known in the lithography field is vector scan writing wherein the beam is positioned only over those sites that require exposure and then unblanked to expose the site. Positioning is accomplished by a combination of the stage and beam movement in what is referred to as a semi-random scan. Pattern data must be provided that includes bout the dose and position of each flash or site exposed.
Frequently vector scan strategies use a variable shaped beam, that is a beam capable of having different size and/or shape, in terms of its cross section, for each flash. The pattern is then composed from these variable shapes. For an example of variable shaped beam see Rishton et al., xe2x80x9cRaster Shaped Beam Electron Beam Exposure Strategy Using a Two-Dimensional Multi Pixel Flash Fieldxe2x80x9d, U.S. patent application Ser. No. 09/226,361, filed Jan. 6, 1999, and commonly owed with the present disclosure, the disclosure of which is incorporated herein by reference in its entirety. The shaped beam in that disclosure is accomplished by associated circuitry and software including shape codes which specify rectangular shaped exposed areas with four different rotations. Various shape codes specify exposed areas that can be either square or rectangular shaped with four different rotations. Other codes represent L-shaped exposed areas with four different rotations. In other embodiments, shape coudes can represent other shapes of the beam. By shape is meant the cross section of the beam where it is incident on the substrate being exposed.
Such a system includes typically a shaper/blanker driver that includes a translator, output device, timer, and can include a retrograde scan device. The shaper/blanker driver request and receives flash data, i.e., shape data and corresponding dose values from respective flash converter and dose value circuitry. A translator receives the flash data and converts the shape data in corresponding dose values into respective voltage values and an exposure time. The translator provides exposure time to a timer and provides both values to an output device.
Additionally, in one embodiment for each input shape data there is an entry in a shape lookup table in a memory and which outputs four voltage values, to various multiplexers. The voltage values specify a two-dimensional electric field deflection by an upper deflector (this is in the context of an electron beam) that effectively control a shaping of the electron beam cross section by controlling a location that the electron beam intersects a lower aperture. Two voltage values specify a two-dimensional electric field deflection by a lower deflector that effectively offsets any deflection by the upper deflector and positions the shaped electron beam on an intended portion of the target substrate. The location at which the electron beam intersects the lower aperture is adjustable by a large number of incremental distance units in either the horizontal or vertically directions within the plane of the lower aperture. This fine incremental positioning allows for offsetting fine errors due, for example, to variations in an opening defined by the lower aperture over time (aperture erosion).
Furthermore, in one embodiment a retrograde scan device adjusts the voltages provided to the lower deflector to offset the movement of the position of the beam on the substrate during the raster scan. The retrograde scan prevents the electron beam column from spreading a flash field beyond its intended area. Further details are in the above-referenced disclosure. Of course, that disclosure is not limiting as to shaped beams. Shaping of laser beams cannot be done using deflectors but instead is accomplished using various aperture arrangements and optics components. As examples, laser beams can be shaped by mechanically changing the shape of an aperture, or by deflecting the laser beam across an aperture (in analogy to the procedure described for electron beams).
Also well known in the electron beam lithography area is raster gray beam writing used in lithography tools known commercially as MEBES(copyright)((manufacturing electron beam exposure system) available from Etec Systems, Inc. See also U.S. Pat. No. 3,900,737 to Collier et al. Examples are the Etec MEBES(copyright) 4500, 5000 and 5500 electron beam lithography systems. For an example of a similar type raster scan electron beam system see Abboud et al. U.S. Pat. No. 5,393,987 issued Feb. 28, 1995, assigned to Etec Systems, Inc. and incorporated herein by reference in its entirety.
Raster gray beam writing refers to use of beams which have graduations of dose intensity on a pixel-by-pixel basis. In addition, gray beam writing can also be achieved by scanning the substrate several times. The flash data can vary from one scan to the next, in order to create the desired gradient on the substrate. This provides benefits in terms of faster write time and improved position-related accuracy of the features being exposed by the beam.
An undesirable effect called edge blur is caused by such gray scale writing techniques. This refers to blurring of the edges of features as exposed. This effect has been minimized in the past by using high contrast resist as the sensitive (resist) layer and a dry etch process for subsequent processing steps. However, there still remain the problems of corner rounding, line end shortening and line edge roughening. Corner rounding refers to features, which typically are intended to be square or rectangular, having their nominally right angle corners rounded off after the substrate is exposed and processed. Line and shortening refers to lines that are not imaged at their normal length. Line edge roughening refers to slanted feature edges, which are lines lying at an angle to the X-Y grid defined by the pixels, not being imaged exactly in their nominal position.
There is a need for sharper corners and more precisely defined lines, especially when making masks. Masks need to be extremely precise because they are later used for producing, using optical stepper lithography, semiconductor integrated circuits. Current technology does not address the corner rounding or line end shortening problems.
This disclosure is directed to a raster scanning method and associated apparatus using multiple pass writing to expose a particular pattern whereby the raster scan is repeated several times. This by itself is well known. Additionally, in accordance with this disclosure, the shape (cross section) of the exposing beam is varied. Certain of the passes are performed using a conventional round Gaussian (diffused) beam. Others of the passes are performed using a shaped beam; each pass images particular pixels. In any one pass, only the shaped beam or Gaussian beam is used. The beam spot size, shape and dose can be changed in each pass. The choice of beam parameters is predetermined and is related to the number of passes and other conventional lithography parameters. This process has been found to improve sharpness of corners, to reduce line end shortening and to improve line edge roughness. Typically the bulk of the feature (most of the non-zero pixels) is exposed using the Gaussian beam. Follow up passes are performed using a shaped beam, especially at the corners and slanted lines. This process can be done without substantially degrading throughput compared to using only a Gaussian beam. For the slanted edges and corners, a higher proportion of the pixels are imaged using the shaped beam versus the Gaussian beam.
Further, one can take advantage of per pixel deflection where particular pixels are scanned having a deflection having other than their nominal deflection in order to improve the imaging of feature serifs using the shaped beam. A serif is a particularly shaped extension attached to a larger feature. Serifs are typically rectangular in shape, and have a size that is smaller than the larger feature. Serifs are used to improve image fidelity on masks after processing and can be used to improve lithography on the semiconductor wafer. Per pixel deflection refers to a known raster scan lithography technique for defining an edge of a feature to lie between pixel centers, i.e., to move the edge of a feature off the nominal pixel grid. This is done directly during pixel writing by deflecting the beam so that the pixel centers are displaced by a variable preselected amount from the normal pixel grid center positions. Such per pixel deflection is typically limited to less than plus or minus one half of an address unit.
Thus, a combination of Gaussian and shaped beams is used for various passes to expose a particular pattern. This process is applicable to both laser beam scanning lithography and charged particle (for instance, electron beam) raster scanning lithography. The corresponding apparatus is essentially a conventional laser or electron beam scanning lithography tool, modified by having a beam shaping element (of the type known in the art) and also of course suitable modifications to the data path and/or data handling software to partition the pixels into those to be exposed by the Gaussian beam versus the shaped beam. The relevant distinction here is that the Gaussian beam typically does show some appreciable intensity peak at its center; whereas the shaped beam is of a more uniform intensity over its entire cross section.
A shaped beam typically is capable of exposing multiple pixel sites simultaneously, instead of the single pixel site exposed one at a time in conventional raster scanning. Hence when a variable shape beam is used, the data must additionally include the location, size and shape for each flash. Hence, the distinguishing characteristics of shaped beams versus Gaussian beams include exposing multiple pixel sites in a single flash using the shape beam. The shaped beams are typically considered somewhat slower than conventional raster scanning. This is partly due to delays in the shaped beam in settling of the electronic beam shaping components which are capable of shaping the beam over a wide range of dimensions. Shaped beams generated by laser systems can also suffer from similar speed constraints. Also, current density is generally lower in the shaped beam approach due to the need for the shaped beam to be capable of covering larger areas simultaneously, again leading to lower throughput.
By xe2x80x9cshapedxe2x80x9d it is meant not just fixed in shape but more inclusively having a variably adjustable shape. This is unlike the case with a Gaussian shape beam which has the typical round shape with only variably adjustable width.